How do I use _W64 and __w64 in VC++?

There's such thing as __w64 in Visual C++ 9. I came across it while trying to port my native C++ DLL to 64 bit. In particular in crtdefs.h there's this nice snippet:

#if !defined(_W64)
#if !defined(__midl) && (defined(_X86_) || defined (_M_IX86)) && _MSC_VER >= 1300
#define _W64 __w64
#else
#define _W64
#endif
#endif

which if I get it right implies that for 64 bit _W64 is defined as an empty string and has no effect, but for 32 bit it is defined as __w64.

I tried defining _W64 to be an empty string and __w64开发者_如何学C in turn and at least both time the project compiles fine in 64-bit configuration.

That said both _W64 and __w64 look useless. How am I intended to use them and how should I define _W64 in 64-bit projects?

__w64 was used to mark types that have different sizes in 32bit and 64bit environments. The compiler could then issue a warning when you assigned a maybe-64bit-type to a 32bit-type.

If you switch the MSDN: __w64 view to the newest version, you can see that the keyword is deprecated. VC9 even generates a warning when you try to compile with the /Wp64 option.

The answer is, you don't. It's a compiler feature for use by the 32 bits compiler. The original idea was that Microsoft wanted to prepare programmers for the upcoming 32->64 bits transition. Therefore, the 32 bits compiler gained the ability to label certain typedefs as __w64. Microsoft then used that ability to label typedefs that would change in Win64.